JavaScript運(yùn)算符:
- 運(yùn)算符 = 用于賦值
- 運(yùn)算符 + 用于加值
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
</head>
<body>
<p>點(diǎn)擊按鈕計(jì)算 x 的值.</p>
<button onclick="myFunction()">點(diǎn)擊這里</button>
<p id="demo">123</p>
<script>
function myFunction()
{
y=5;
z=2;
x=y+z;
document.getElementById("demo").innerHTML=x;
}
</script>
</body>
</html>
算數(shù)運(yùn)算符
+ - * / % ++ --
賦值運(yùn)算符
= += -= *= /= %=
用于字符串的 + 運(yùn)算符
+
運(yùn)算符用于把文本值或字符串變量加起來(連接起來)嚷节。
如需把兩個(gè)或多個(gè)字符串變量連接起來,請(qǐng)使用+
運(yùn)算符狈谊。
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>菜鳥教程(runoob.com)</title>
</head>
<body>
<p>點(diǎn)擊按鈕創(chuàng)建及增加字符串變量钻蹬。</p>
<button onclick="myFunction()">點(diǎn)擊這里</button>
<p id="demo"></p>
<script>
function myFunction()
{
txt1="What a very";
txt2="nice day";
txt3=txt1+txt2;
document.getElementById("demo").innerHTML=txt3;
}
</script>
</body>
</html>
對(duì)字符串和數(shù)字進(jìn)行加法運(yùn)算
兩個(gè)數(shù)字相加崖蜜,返回?cái)?shù)字相加的和,如果數(shù)字與字符串相加,返回字符串
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>菜鳥教程(runoob.com)</title>
</head>
<body>
<p>點(diǎn)擊按鈕創(chuàng)建及增加字符串變量虏杰。</p>
<button onclick="myFunction()">點(diǎn)擊這里</button>
<p id="demo"></p>
<script>
function myFunction()
{
var x=5+5;
var y="5"+5;
var z="Hello"+5;
var demoP=document.getElementById("demo");
demoP.innerHTML=x + "<br>" + y + "<br>" + z;
}
</script>
</body>
</html>
比較運(yùn)算符
== === != !== > < >= <=
===
為絕對(duì)等于,值和類型均相等才返回true;
==
值相等即返回true, 比如5 == "5"返回true.
!==
不絕對(duì)等于(值和類型有一個(gè)不相等蛮艰,或兩個(gè)都不相等)
邏輯運(yùn)算符
&& || !
分別為 and or not
條件語(yǔ)句
if...else語(yǔ)句
- if
- if...else
- if...else if ...else
- switch
<html>
<head>
<meta charset="utf-8">
<title>菜鳥教程(runoob.com)</title>
</head>
<body>
<p>點(diǎn)擊下面的按鈕腋腮,會(huì)顯示出基于今日日期的消息:</p>
<button onclick="myFunction()">點(diǎn)擊這里</button>
<p id="demo"></p>
<script>
function myFunction()
{
var x;
var d=new Date().getDay();
switch (d)
{
case 6:x="今天是星期六";
break;
case 0:x="今天是星期日";
break;
default:
x="期待周末";
}
document.getElementById("demo").innerHTML=x;
}
</script>
</body>
</html>
循環(huán)
- for
- for/in
- while
- do while
for/in 語(yǔ)句循環(huán)遍歷對(duì)象的屬性
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>菜鳥教程(runoob.com)</title>
</head>
<body>
<p>點(diǎn)擊下面的按鈕,循環(huán)遍歷對(duì)象 "person" 的屬性壤蚜。</p>
<button onclick="myFunction()">點(diǎn)擊這里</button>
<p id="demo"></p>
<script>
function myFunction(){
var x;
var txt="";
var person={fname:"Bill",lname:"Gates",age:56};
for (x in person){
txt=txt + person[x];
}
document.getElementById("demo").innerHTML=txt;
}
</script>
</body>
</html>
break continue語(yǔ)句
continue 語(yǔ)句只能用在循環(huán)中;
break 語(yǔ)句(不帶標(biāo)簽引用)即寡,只能用在循環(huán)或 switch 中;
JavaScript標(biāo)簽
如需標(biāo)記 JavaScript 語(yǔ)句,請(qǐng)?jiān)谡Z(yǔ)句之前加上冒號(hào)
通過標(biāo)簽引用袜刷,break 語(yǔ)句可用于跳出任何 JavaScript 代碼塊
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>菜鳥教程(runoob.com)</title>
</head>
<body>
<script>
cars=["BMW","Volvo","Saab","Ford"];
list:{
document.write(cars[0] + "<br>");
document.write(cars[1] + "<br>");
document.write(cars[2] + "<br>");
break list;
document.write(cars[3] + "<br>");
document.write(cars[4] + "<br>");
document.write(cars[5] + "<br>");
}
</script>
</body>
</html>
Typeof 操作符來檢測(cè)變量的數(shù)據(jù)類型
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>菜鳥教程(runoob.com)</title>
</head>
<body>
<p> typeof 操作符返回變量或表達(dá)式的類型聪富。</p>
<p id="demo"></p>
<script>
document.getElementById("demo").innerHTML =
typeof "john" + "<br>" +
typeof 3.14 + "<br>" +
typeof false + "<br>" +
typeof [1,2,3,4] + "<br>" +
typeof {name:'john', age:34};
</script>
</body>
</html>
- 在JavaScript中,數(shù)組是一種特殊的對(duì)象類型, 因此 typeof [1,2,3,4] 返回 object;
- null是一個(gè)只有一個(gè)值的特殊類型著蟹。表示一個(gè)空對(duì)象引用.
用 typeof 檢測(cè) null 返回是object墩蔓。
var person = null; // 值為 null(空), 但類型為對(duì)象
- 設(shè)置為 undefined 來清空對(duì)象.
在 JavaScript 中, undefined 是一個(gè)沒有設(shè)置值的變量。
typeof 一個(gè)沒有值的變量會(huì)返回 undefined萧豆。
var person = undefined; // 值為 undefined, 類型為 undefined
person = undefined; // 值為 undefined, 類型是undefined
- undefined和null區(qū)別:
null 和 underfined 的值相等奸披,但類型不等
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>菜鳥教程(runoob.com)</title>
</head>
<body>
<p id="demo"></p>
<script>
document.getElementById("demo").innerHTML =
typeof undefined + "<br>" +
typeof null + "<br>" +
(null === undefined) + "<br>" +
(null == undefined);
</script>
</body>
</html>